For measuring a target point, numerous geodetic measuring devices have been known since ancient times. In this case, distance and direction or angle from a measuring device to the target point to be measured are recorded and, in particular, the absolute position of the measuring device together with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodetic measuring devices include the theodolite, tachymeter and total station, which is also designated as electronic tachymeter or computer tachymeter. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such devices have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the device and, if appropriate, also have to be combined with an external reference system for absolute position determination.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The devices generally have a compact and integrated design, wherein coaxial distance measuring elements and also computing, control and storage units are usually present in a device. Depending on the expansion stage of the total station, motorization of the targeting or sighting device and means for automatic target seeking and tracking can additionally be integrated. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data detected in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit. Total stations known from the prior art can furthermore have a radio data interface for setting up a radio link to external peripheral components such as e.g. a handheld data acquisition device, which can be designed, in particular, as a data logger or field computer.
For sighting or targeting the target point to be measured, geodetic measuring devices of the generic type have a telescopic sight such as e.g. an optical telescope, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the measuring device, such that the telescopic sight can be aligned with the point to be measured by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for detecting an image, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion, wherein the detected image can be represented, in particular, as a live image on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger—used for remote control. In this case, the optical system of the sighting device can have a manual focus—for example an adjusting screw for altering the position of a focusing optical system—or an autofocus, wherein the focus position is altered e.g. by servomotors. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 19710722, DE 19926706 or DE 19949580.
The optical system or the optical viewing channel of the sighting device usually contains an objective lens group, an image reversal system, a focusing optical system, a reticule for producing a cross hair and an eyepiece, which are arranged e.g. in this order from the object side. The position of the focusing lens group is set depending on the object distance in such a way that a sharp object image arises on the reticule arranged in the focusing plane. Said image can then be viewed through the eyepiece or e.g. detected with the aid of a camera arranged coaxially.
By way of example, the construction of generic telescopic sights of geodetic devices is disclosed in the publication documents EP 1 081 459 or EP 1 662 278.
On account of the beam path that is usually to be utilized jointly both as viewing channel and for measurements, such devices require the technical design of said beam path in the manner of construction of a telescope with specialized, high-precision optical systems that are to be produced with a high outlay. Furthermore, an additional separate transmitting and receiving channel and also an additional image plane for the wavelength of the distance measuring device are provided for the coaxial electronic distance measurement. Moreover, conventional measuring devices in the meantime have an automatic target tracking function (ATR: “Automatic Target Recognition”), for which a further separate ATR light source—e.g. multimode fiber output, which emits light having a wavelength of 850 nm—and also a specific ATR camera sensor are additionally integrated in the telescope, such that the telescope optical system has a high complexity with some ramified beam paths.
In order to avoid distortions, color casts or vignetting—that is to say a decrease in brightness in edge regions of the observable field of view—extremely stringent requirements are made of the individual optical components. Accordingly, optical units that are coated specifically and in a manner involving a high outlay are necessary for coupling individual wavelengths out and in, in which case, despite the coating, the visual band is intended to allow a representation exhibiting the highest possible color fidelity. Moreover, the high complexity of the telescope necessitates a high outlay for the required high-precision mounting and adjustment of the optical components.
To summarize, as a result of the optical components of the telescope being utilized jointly both for the direct viewing channel and for measurements, the requirements made of the optical components are very stringent, which—disadvantageously—necessitates a high outlay in the production, mounting and adjustment of the optical components.
In order to keep down the risk of a parallax error between the direct-vision image and the reticule producing the crosshair when the user looks into the eyepiece obliquely, the aperture of the exit pupil of the eyepiece is very small in the generic telescopic sights. By way of example, the aperture of the pupil has a diameter of just a few millimeters. This also proves to be disadvantageous, since viewing the target through the small pupil aperture is found to be strenuous by users and is suboptimal particularly for users who wear spectacles. This is because the field of view for the observer is already significantly restricted when looking into the eyepiece at a slight distance from the eyepiece.
In order to allow the user to experience more relaxed viewing of the target to be measured and an ergonomically expedient application of the measuring device, camera-screen combinations are often being used in the meantime. For this purpose, by way of example, a coaxial camera is additionally provided in the telescopic sight.
The camera image can be represented on the user display of the display control unit or on the display of a separate peripheral device (e.g. data logger), such that control of the measuring process and also target identification or target tracking and hence facilitation and automation of the measuring process can be effected.
However, the sighting of the target object by the user is usually still effected by means of viewing the target through the eyepiece of the telescopic sight, since the displayed display image of the display control unit or of the data logger may be insufficiently recognizable during use in practice—e.g. in the case of insolation.
Moreover, the camera present in addition to the direct viewing channel requires a further image plane provided in the telescope optical system for the camera and coupling-out of a partial light beam via a beam splitter, which further increases the complexity of the telescopic sight and reduces the brightness of the direct viewing channel image.